I am simply building a Python container with about ~15 dependencies installed via pip and some ~50 MB of static assets added to the container. The base image comes from Docker Hub (hosted on S3/AWS CF).
If the final destination of your Docker image is GCR, it is really fast. You're basically not waiting for your build box to start up. Based on my experience the build starts almost right away after you push to the source control system, whereas CI systems like TravisCI/CircleCI spend quite some time preparing the build env (i.e. container) and running some preliminary commands, collecting artifacts/logs etc. Google Container Builder is looks like it is designed to build containers (not to run tests) and it does that quite fast.
If the final destination of your Docker image is GCR, it is really fast. You're basically not waiting for your build box to start up. Based on my experience the build starts almost right away after you push to the source control system, whereas CI systems like TravisCI/CircleCI spend quite some time preparing the build env (i.e. container) and running some preliminary commands, collecting artifacts/logs etc. Google Container Builder is looks like it is designed to build containers (not to run tests) and it does that quite fast.